This invention relates to the measurement of vacuum pressure levels and is especially useful for the purpose of determining the vacuum integrity of microwave vacuum tubes and other compact devices. Many microwave tube failures result from degradation of vacuum levels within the tube. Frequently, tubes remain on storage shelves for months or years, losing vacuum integrity during that time. Much time and expense may be expended in installing a microwave tube only to discover that the tube is not viable. This invention provides a method by which vacuum tubes can be tested prior to their installation. This invention also provides a small vacuum gauge for general vacuum testing.
Techniques exist for the measurement of vacuum levels which operate on the basic principles of collection of ionized residual gas molecules or the cooling of a hot filament placed within the vacuum tube. Examples of vacuum gauges using either of these two methods are mass spectrometers, Bayard-Alpert gauges, hot cathodes ionization gauges, Penning or Philips gauge, Schulz-Phelps gauges, or Pirani gauges.
The existing gauges utilizing the method of collection of ionized residual gas molecules are particularly valuable for the sensitivity which they afford in use. However, some sensitivity is lost with miniaturization of the gauge which necessarily reduces the electron path length associated with such gauges. Gauges utilizing the method of cooling of a hot filament by residual gas molecules are simple in design and are easily miniaturized. However, such gauges are not highly sensitive, and likely to further lose sensitivity with miniaturization.
Usual gas ionization vacuum gauges use a hot or cold cathode to emit an electron current. Ionization of residual gas molecules occurs by collision with electrons flowing along the current path to a collector. The chance that an electron will ionize a gas molecule at pressures of 10.sup.-6 torr or lower, the pressure at which failure of the vacuum begins, is small. Accordingly, a decrease in the electron path length in miniaturization will reduce the probability of ionization. As a result, small volume gauges must produce a significantly higher electron current in order to produce a significant ion current.
The invention herein overcomes the attendant problems associated with currently available gauges, as discussed above. The present invention employs a multipactor discharge as the electron current source which allows high electron current densities in a miniaturized gauge.